In the prior art, when telecommunication customers each with, for example, a telephone and a data terminal wanted both voice signal and data signal transmission capability, two separate two-wire communication lines were required to serve each customer. One two-wire line transfers the voice signals, and the other two-wire line with a data set(multiplexer-demultiplexer) at each end of the line transfers the data signals. However, many voice signal switching systems have a limited bandwidth and are unable to pass the data signals. Consequently, a data signal switching system typically switches the data lines, and a separate voice signal switching system switches the voice signal lines. The problem with this arrangement is that two two-wire lines, one with two data sets, for each customer and two switching systems are required to provide customers with both simultaneous and independent voice signal and data signal transmission capability.
Another prior art arrangement utilizes a data set at each end of a single two-wire line for each customer to simultaneously and independently transfer voice and data signals. The data set frequency multiplexes the voice and the data signals at the customer end of the line and demultiplexes the signals at the other end of the line before the separated voice signal is sent to a voice signal switching system. The separated data signal is typically sent to a computer port that is connected to the data signal line on a dedicated basis. The problem with this second arrangement is that a line with two data sets and a computer port for each customer and a voice signal switching system are still required. Depending on the number of data signal lines and the actual use of each line, the cost of a large number of computer ports with low utilization can be enormous.